Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
From Dust to Planetesimals: Implications for the Solar
Protoplanetary Disk from Short-lived Radionuclides
M. Wadhwa
The Field Museum
Y. Amelin
Geological Survey of Canada
A. M. Davis
The University of Chicago
G. W. Lugmair
University of California at San Diego
B. Meyer
Clemson University
M. Gounelle
Muséum National d’Histoire Naturelle
S. J. Desch
Arizona State University
Since the publication of the Protostars and Planets IV volume in 2000, there have been
significant advances in our understanding of the potential sources and distributions of shortlived, now extinct, radionuclides in the early solar system. Based on recent data, there is definitive evidence for the presence of two new short-lived radionuclides (10Be and 36Cl) and a
compelling case can be made for revising the estimates of the initial solar system abundances
of several others (e.g., 26Al, 60Fe, and 182Hf). The presence of 10Be, which is produced only by
spallation reactions, is either the result of irradiation within the solar nebula (a process that
possibly also resulted in the production of some of the other short-lived radionuclides) or of
trapping of galactic cosmic rays in the protosolar molecular cloud. On the other hand, the latest estimates for the initial solar system abundance of 60Fe, which is produced only by stellar
nucleosynthesis, indicate that this short-lived radionuclide (and possibly significant proportions
of others with mean lives ≤10 m.y.) was injected into the solar nebula from a nearby stellar
source. As such, at least two distinct sources (e.g., irradiation and stellar nucleosynthesis) are
required to account for the abundances of the short-lived radionuclides estimated to be present
in the early solar system. In addition to providing constraints on the sources of material in the
solar system, short-lived radionuclides also have the potential to provide fine-scale chronological
information for events that occurred in the solar protoplanetary disk. An increasing number of
studies are demonstrating the feasibility of applying at least some of these radionuclides as
high-resolution chronometers. From these studies, it can be inferred that the millimeter- to
centimeter-sized refractory calcium-aluminum-rich inclusions in chondritic meteorites are among
the earliest solids to form (at 4567.2 ± 0.6 Ma). Formation of chondrules (i.e., submillimetersized ferromagnesian silicate spherules in chondrites) is likely to have occurred over a time
span of at least ~3 m.y., with the earliest ones possibly forming contemporaneously with CAIs.
Recent work also suggests that the earliest planetesimals began accreting and differentiating
within a million years of CAI formation, i.e., essentially contemporaneous with chondrule formation. If so, it is likely that undifferentiated chondrite parent bodies accreted a few million
years thereafter, when the short-lived radionuclides that served as the main heat sources for
melting planetesimals (26Al and 60Fe) were nearly extinct.
835
835
836
Protostars and Planets V
1.
INTRODUCTION
Short-lived radionuclides are characterized by half-lives
(T½) that are significantly shorter (i.e., ≤100 m.y.) than the
4.56-Ga age of the solar system. Although now extinct, their
former presence at the time of solar system formation can
be inferred if variations in their daughter isotopes are demonstrated to correlate with parent/daughter element ratios
in meteorites and their components. These radionuclides are
of particular interest since (1) an understanding of their
sources and distributions in the early solar system (ESS)
can provide constraints on the formation environment and
astrophysical setting of the solar protoplanetary disk and
(2) they have the potential for application as fine-scale chronometers (in many cases with a time resolution of ≤1 m.y.)
for events occurring in the early history of the solar system.
A prerequisite for the application of a fine-scale chronometer based on a short-lived radionuclide is that the initial abundance of this radionuclide must be demonstrated
to be uniform in the region of the solar system where rocky
bodies were forming. Moreover, since the slope of an isochron derived from such a chronometer provides not an age
but a measure of the abundance of the radionuclide at the
time of last isotopic closure, comparison of the isochron
slopes for two separate events can provide only a relative
time difference between these events. For such high-resolution relative ages to be mapped on to an absolute timescale,
they need to be pinned to a precise time “anchor,” which is
usually provided by the U-Pb chronometer (which is capable of providing absolute ages with a precision comparable
to that of the short-lived chronometers). Further details on
the application of short-lived radionuclides as chronometers
and the caveats involved have been discussed in several review articles (e.g., Wasserburg, 1985; Podosek and Nichols,
1997; Wadhwa and Russell, 2000; McKeegan and Davis,
2003; Kita et al., 2005; Gounelle and Russell, 2005).
The purpose of this review is not to provide a comprehensive overview of short-lived radionuclides and their application to the study of meteorites and their components
(which may be found in several of the reviews mentioned
above). Instead, we will focus on the most recent results and
the advances in our understanding of the sources and distributions of these radionuclides since the publication of Protostars and Planets IV (PPIV). In the following sections, we
will discuss their two main potential sources, i.e., stellar
nucleosynthesis and local production by irradiation. Furthermore, based on our current understanding of the abundances
and distributions of short-lived radionuclides in the ESS,
we will discuss the implications for the astrophysical setting
and for the timing of events from “dust to planetesimals”
in the solar protoplanetary disk.
2. SHORT-LIVED RADIONUCLIDES IN
THE EARLY SOLAR SYSTEM:
THE LATEST RESULTS
Table 1 provides a listing of the short-lived radionuclides
for which there is now definitive evidence of their former
presence in the ESS, although the initial solar system abun-
TABLE 1. Short-lived radionuclides
in the early solar system.
Parent
Isotope
10Be
26Al
36Cl
Daughter
Isotope
T½ *
1.5
0.72
0.3
10B
26 Mg
36Ar
(98.1%)
(1.9%)
41K
53 Cr
60 Ni
92 Zr
107 Ag
129Xe
142 Nd
182W
Fission Xe
36S
41Ca
53 Mn
60Fe
92Nb
107Pd
129I
146Sm
182Hf
244Pu
0.1
3.7
1.5
36
6.5
15.7
103
8.9
82
Solar System
Initial Abundance†
≈ 10 –3
≈ 5–7 × 10 –5
36Cl/35Cl ≥ 1.6 × 10 –4
10 Be/9Be
26 Al/27Al
≥ 1.5 × 10 –8
≈ 10 –5
60Fe/56Fe ≈ 3–10 × 10 –7
92Nb/ 93Nb ≈ 10 –5–10 –3
107 Pd/108Pd ≈ 5–40 × 10 –5
129I/129 Xe ≈ 10 –4
146 Sm/144 Sm ≈ 7 × 10 –3
182 Hf/180Hf ≈ 10 –4
244Pu/238 U ≈ 7 × 10 –3
41 Ca/40Ca
53 Mn/55Mn
*Half-life in millions of years.
† Data sources: 10Be: McKeegan et al. (2000); 26Al: Lee et al.
(1976), MacPherson et al. (1995), Bizzarro et al. (2004),Young
et al. (2005); 36Cl: Lin et al. (2005); 41Ca: Srinivasan et al.
(1994, 1996); 53Mn: Lugmair and Shukolyukov (1998); 60Fe:
Tachibana and Huss (2003), Mostefaoui et al. (2005), Tachibana et al. (2006); 92Nb: Harper (1996), Münker et al. (2000),
Sanloup et al. (2000), Yin et al. (2000), Schönbächler et al.
(2002); 107Pd: Chen and Wasserburg (1996), Carlson and Hauri
(2001); 129I: Swindle and Podosek (1988) and references therein,
Brazzle et al. (1999); 146Sm: Lugmair and Galer (1992) and references therein; 182Hf: Kleine et al., (2002, 2005), Yin et al.
(2002); 244Pu: Podosek (1970), Hudson et al. (1989).
dances of some of these are somewhat uncertain. Several
others, such as 7Be (T½ = 53 d) (Chaussidon et al., 2006),
99Tc (T ~ 0.2 m.y.) (Yin et al., 1992), 135Cs (T ~ 2.3 m.y.)
½
½
(Hidaka et al., 2001) and 205Pb (T½ ~ 15 m.y.) (Chen and
Wasserburg, 1987; Nielsen et al., 2004), may also have been
present but evidence for these is as yet suggestive rather
than definitive.
Since PPIV, two new short-lived radionuclides (10Be and
36Cl) have been added to the roster of those for which there
is now compelling evidence for their former presence in the
ESS (Table 1). In addition, the presence of 92Nb, for which
there was only suggestive evidence prior to 2000, has been
confirmed by several recent studies, although its initial abundance is still debated. Also, on the basis of recent analyses
of meteorites and their components, the initial abundances
of several of the short-lived radionuclides listed in Table 1
have been revised. Some of the implications of these new
results will be discussed in the following sections.
2.1.
Beryllium-10
McKeegan et al. (2000) showed that excesses in 10B/11B
are correlated with 9Be/11B ratios in calcium-aluminum-rich
inclusions (CAI) from the Allende carbonaceous chondrite,
indicating an initial 10Be/9Be ratio of ~10 –3 in the ESS
(Fig. 1). Subsequently, additional studies of CAIs from other
chondrite groups have confirmed this finding (Sugiura et
al., 2001; Marhas et al., 2002; MacPherson et al., 2003).
Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
837
clasts (Stewart et al., 1994) may indeed record late disturbances >100 m.y. after the beginning of the solar system,
there is no definitive indicator that the Nb-Zr system was
reset in these samples at this time. As such, the initial abundance of 92Nb in the ESS is as yet unclear.
2.4. Initial Abundances of Aluminum-26,
Calcium-41, Iron-60, and Hafnium-182
Fig. 1. Boron-isotopic composition of Allende CAIs vs. Be/B
ratios (McKeegan et al., 2000).
2.2.
Chlorine-36
Recently, Lin et al. (2005) presented new evidence (excesses in 36S that correlated with Cl/S ratios) for the presence of live 36Cl in sodalite, a chlorine-rich mineral that
most likely formed from aqueous alteration on the parent
body, in a CAI from the Ningqiang carbonaceous chondrite
(Fig. 2). This work indicates that the 36Cl/35Cl ratio in the
ESS was at least ~1.6 × 10 –4.
2.3.
The initial abundances of several of the radionuclides
listed in Table 1 have been revised significantly since PPIV.
Until recently, the initial 26Al/27Al ratio in the ESS was
thought to have the canonical value of ~5 × 10–5 (Lee et al.,
1976; MacPherson et al., 1995). However, recent high-precision Mg-isotopic analyses of CAIs indicate that the initial 26Al/27Al ratio may have been as high as 6–7 × 10 –5
(Bizzarro et al., 2004, 2005a; Young et al., 2005; Taylor et
al., 2005). If this higher value for the initial 26Al/27Al ratio
is assumed, then the initial 41Ca/40Ca ratio may also have
been correspondingly higher than the previously inferred
value of ~1.5 × 10 –8 by at least an order of magnitude,
because the initial 41Ca/40Ca ratio was measured on CAIs
with internal isochrons indicating an initial 26Al/27Al ratio
of ~5 × 10 –5.
Although Birck and Lugmair (1988) had noted excesses in 60Ni in Allende CAIs, these could be attributable to
nucleosynthetic anomalies and the first definitive evidence
for the presence of live 60Fe in the ESS came from the
work of Shukolyukov and Lugmair (1993a,b). These authors
showed that excesses in 60Ni were correlated with Fe/Ni
ratios in bulk samples of the eucrites Chervony Kut and
Juvinas. Based on their analyses, Shukolyukov and Lugmair
Niobium-92
Until just a few years ago, the only hint of the former
presence of 92Nb in the ESS had been a well-resolved excess of 92Zr in rutile, a rare mineral with a high Nb/Zr ratio, in the Toluca iron meteorite, based upon which an initial 92Nb/93Nb ratio of ~2 × 10–5 was inferred for the solar
system (Harper, 1996). Subsequently, several studies also
reported excesses in 92Zr in bulk samples and mineral separates from a variety of primitive and differentiated meteorites, but suggested a substantially higher initial 92Nb/93Nb
ratio of ~10 –3 (Münker et al., 2000; Sanloup et al., 2000;
Yin et al., 2000). However, Schönbächler et al. (2002) reported internal 92Nb-92Zr isochrons for the H6 chondrite
Estacado and a basaltic clast from the Vaca Muerta mesosiderite and inferred an initial solar system 92Nb/93Nb ratio
of ~10–5. Yin and Jacobsen (2002) have suggested that Estacado and the Vaca Muerta clast may record secondary events
that postdated solar system formation by ~150 ± 20 m.y. If
so, the lower 92Nb/93Nb ratio inferred by Schönbächler et
al. (2002) would be compatible with the higher value of
~10 –3 (which would then reflect the true initial value) reported by others (Münker et al., 2000; Sanloup et al., 2000;
Yin et al., 2000). Although the 40Ar-39Ar ages for Estacado
(Flohs, 1981) and the 147Sm-143Nd ages for Vaca Muerta
Fig. 2. Sulfur-isotopic compositions of sodalite-rich assemblages
in a Ningqiang CAI (Lin et al., 2005). The best inferred 36Cl/35Cl
ratio from these data is 5 × 10 –6. Assuming that the sodalites
formed ≥1.5 m.y. after CAIs, an initial 36Cl/35Cl ratio of ≥1.6 ×
10 –4 is inferred.
838
Protostars and Planets V
–3.5ε units inferred from chondrites and their components,
may be due to burnout of W isotopes from long exposure to
galactic cosmic rays (GCRs) (Markowski et al., 2006; Qin
et al., 2005). Two recent Hf-W studies of meteoritic zircons
(which are good candidates for Hf-W chronometry owing
to their typically high Hf/W ratios) provide somewhat conflicting results. Ireland and Bukovanská (2003) confirmed
an initial 182Hf/180Hf ratio based on Hf-W systematics in
zircons from the H5 chondrite Simmern of close to ~1 ×
10 –4. These authors also reported Hf-W systematics in zircons from the Pomozdino eucrite that gave a substantially
lower 182Hf/180Hf ratio of ~2 × 10–5, perhaps suggestive of
late-metamorphic resetting in this eucrite. In contrast, Srinivasan et al. (2004), who analyzed Hf-W and U-Pb systematics in zircons from another eucrite, have suggested the initial 182Hf/180Hf ratio was at least ~3 × 10–4. The reason for
these apparently discrepant values for the initial 182Hf/180Hf
ratio is not clear. Nevertheless, the most recent work on the
W isotopes in CAIs appears to support an initial 182Hf/180Hf
ratio of ~1 × 10–4 (Kleine et al., 2005).
Fig. 3. Excesses in the 60Ni/62Ni ratio relative to a terrestrial standard in parts per 103 vs. Fe/Ni ratios in troilites from metal-free
assemblages in the Semarkona unequilibrated ordinary chondrite
(Mostefaoui et al., 2005). The slope of the Fe-Ni isochron yields
a 60Fe/56Fe ratio of (0.92 ± 0.24) × 10 –6.
3. SOURCES OF SHORT-LIVED
RADIONUCLIDES AND THEIR
IMPLICATIONS
3.1.
(1993a,b) inferred an initial 60Fe/56Fe ratio of ~10 –8. Recently, ion microprobe analyses of components in unequilibrated chondrite meteorites indicate that the initial solar
system 60Fe/56Fe ratio is likely to be as high as ~10–6 (Tachibana et al., 2006; Mostefaoui et al., 2005) (Fig. 3). The
lower initial 60Fe/56Fe ratio inferred from the eucrites (which
are known to have undergone varying degrees of thermal
metamorphism) is thought to be the result of partial equilibration of the Fe-Ni system.
The earliest estimates of an upper limit on the initial
182Hf/180Hf ratio for the ESS based on analyses of meteoritic material suggested that it was ≤~2 × 10–4 (Ireland, 1991;
Harper and Jacobsen, 1996). Subsequently, the work of Lee
and Halliday (1995, 1996) indicated that the W isotope
composition of the bulk silicate Earth (BSE) was identical
to that of the chondrites and the initial 182Hf/180Hf ratio for
the ESS was ~2.5 × 10 –4. However, several recent studies
demonstrated that the W isotope composition of the BSE
is more radiogenic than chondrites by ~2ε units (Kleine et
al., 2002; Schoenberg et al., 2002; Yin et al., 2002) and
indicated a lower initial 182Hf/180Hf ratio of ~1 × 10–4 (note
that W isotope composition in ε units or ε182W is defined
as the 182W/183W or the 182W/184W ratio relative to the terrestrial standard in parts per 10 4). Based on the extremely
unradiogenic W isotope composition of the Tlacotepec iron
meteorite, Quitté and Birck (2004) suggested an intermediate value of ~1.6 × 10 –4 for the initial 182Hf/180Hf ratio.
However, the highly unradiogenic ε182W values reported in
some iron meteorites, i.e., lower than the initial value of
Stellar Nucleosynthesis
Most of the short-lived radioactive nuclides present in
the ESS could be produced and ejected from stars. In this
section we briefly review the stellar synthesis of these isotopes and discuss possible implications for their distribution in the solar nebula.
3.1.1. Aluminum-26. Aluminum-26 is produced in
hydrogen burning by the reactions 25Mg(p,γ) 26Al and
26Mg(p,n)26Al. Such 26Al may be ejected from dredged-up
hydrogen-shell-burning material in low-mass stars (stars
with masses less than about 8 M ) during their red giant
branch (RGB) or asymptotic giant branch (AGB) phases or
from high-mass stars (stars with masses greater than roughly
10 M ) when they explode as core-collapse supernovae.
This radioisotope is also made during carbon burning as
neutrons and protons liberated in the fusion of two carbon
nuclei drive capture reactions on Mg isotopes. Carbon-burning-produced 26Al would only be ejected from massive stars.
3.1.2. Chlorine-36 and calcium-41. The isotopes 36Cl
and 41Ca are synthesized in s-process nucleosynthesis, predominantly during core helium burning in massive stars.
Production of 36Cl and 41Ca also occurs during explosive
oxygen burning in supernova events.
3.1.3. Manganese-53. Manganese-53 is produced predominantly in silicon burning with some contribution from
oxygen burning. While production of 53Mn does occur in the
presupernova evolution of a massive star, most of the 53Mn
ejected is synthesized during the explosive phase as the
shock wave generated by the stellar collapse passes through
silicon- and oxygen-rich layers of the star. These layers lie
Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
near the boundary between what escapes the star and what
remains behind in the neutron star or black hole resulting
from the explosion. A roughly comparable amount is made
in typical Type Ia supernovae, which are thermonuclear explosions of white dwarf stars. Manganese-53 is not produced in low-mass stars.
3.1.4. Iron-60. Iron-60 cannot be produced in any significant amount during mainline s-processing since the short
lifetime of 59Fe prevents much neutron capture flow to
heavier isotopes of Fe. The later stage of carbon burning
achieves a higher neutron density and allows for significant
production of 60Fe, a fact that strongly favors massive stars
as the site of production of this isotope. Most of the 60Fe
ejected from stars, however, is likely produced in explosive
carbon burning during the supernova explosion with significant production also occurring in the neutron burst in the
helium shell (e.g., Meyer, 2005). Significant production may
occur in higher-mass AGB stars (e.g., Gallino et al., 2004)
and some may even occur in the rare deflagrating or detonating white dwarf stars that likely produced the bulk of the
solar system’s supply of 48Ca (Meyer et al., 1996; Woosley,
1997), but the yield from these events is not yet certain.
3.1.5. Palladium-107, iodine-129, hafnium-182, and
plutonium-244. The isotopes 107Pd, 129I, 182Hf, and 244Pu
are produced by the r-process of nucleosynthesis whose site
is not yet determined. The most promising setting for the
r-process is in neutrino-heated ejecta from core-collapse supernovae (e.g., Woosley et al., 1994); however, tidal disruptions of neutron stars have not been ruled out (e.g., Freiburghaus et al., 1999). Palladium-107 is also produced in the
s-process of nucleosynthesis, which occurs when neutrons
liberated by the reaction 13C(α,n)16O and 22Ne(α,n)25Mg are
subsequently captured by heavier seed nuclei. This production may occur in AGB stars during shell helium burning
or in massive stars during core or shell helium burning (Gallino et al., 2004). Importantly, 129I and 182Hf are also produced in the neutron burst that occurs in the inner parts of
the helium-rich shell in the massive star during a supernova
explosion (e.g., Meyer, 2005). This neutron burst contributes
only a small amount of the 129I and 182Hf that has ever existed in the galaxy; however, it may have contributed a dominant portion of the abundance of these isotopes present in
the ESS. It is important to note, however, that a neutron burst
will not produce 244Pu since the seed uranium or thorium
nuclei would have been burned up by s-processing prior to
the neutron burst. Any 244Pu alive in the ESS must have
been a residue of galactic r-process nucleosynthesis.
3.1.6. Niobium-92 and samarium-146. A handful of
heavy, proton-rich nuclei are bypassed by neutron capture
processes. These nuclei are made in a separate process,
known as the p-process, which is thought to occur in corecollapse supernovae as the shock wave passes through the
oxygen-/neon-rich layers of a massive star during the stellar explosion. The heating due to shock passage causes first
neutrons, then protons and α particles to disintegrate from
preexisting seed nuclei. Once this disintegration process
839
freezes out, a distribution of proton-rich nuclei remains.
This synthesis process is also known in the literature as the
γ-process (Woosley and Howard, 1978; Arnould and Goriely,
2003). It is also possible that the γ-process occurs in the
outer layers of a Type Ia supernova if the burning front is
a deflagration when it reaches the surface (Howard et al.,
1991).
While the γ-process can account for the abundance of
heavy p-process nuclei, including 146Sm, it falls short of
producing the light p-nuclei, particularly the p-process isotopes of molybdenum and ruthenium. It is therefore likely
that some other process is responsible for the bulk production of those isotopes, and, concomitantly, for 92Nb. The
most promising process is the freeze-out from high-entropy
nuclear statistical equilibrium near the mass cut of a corecollapse supernova (e.g., Fuller and Meyer, 1995; Hoffman
et al., 1996). Interactions between nuclei and the copious
supply of supernova neutrinos may be necessary to account
for the proper supply of p-process isotopes of molybdenum,
ruthenium, and niobium (e.g., Meyer, 2003; Frölich et al.,
2006).
Table 2 summarizes the above discussion. The nucleosynthesis processes delineated above synthesized the shortlived isotopes over the course of the galaxy’s evolution. A
steady-state abundance of these isotopes developed in the
interstellar medium (ISM) as the rate of production in stars
TABLE 2. Stellar nucleosynthetic processes and
sources of short-lived radionuclides.
Isotope
26 Al
36 Cl
41 Ca
53 Mn
60 Fe
92 Nb
107 Pd
129 I
146 Sm
182 Hf
244 Pu
Nucleosynthesis
Process
Site
Hydrogen burning
Carbon burning
s-process
Oxygen burning
s-process
Oxygen burning
Silicon burning
NSE
Carbon burning
Neutron burst
s-process
Neutron-rich NSE
p-process
r-process
s-process
r-process
Neutron burst
p-process
r-process
Neutron burst
r-process
MS, RGB, AGB
MS
MS, AGB
MS
MS, AGB
MS
MS
SNIa
MS
MS
AGB
Rare SNIa
MS, SNIa
MS, NS
MS, AGB
MS, NS
MS
MS, SNIa
MS, NS
MS
MS, NS
MS = massive star; RGB = red giant branch star; AGB = asymptotic giant branch star; SNIa = Type Ia supernova; NS = neutron
star disruptions; NSE = nuclear statistical equilibrium.
840
Protostars and Planets V
came to balance the rate of destruction by decay and astration. The solar system inherited this ISM abundance. Since
we expect the dust and gas that carried the short-lived radionuclides into the solar cloud to be fairly well mixed, we
would therefore expect a reasonably uniform distribution
of these radionuclides in the protosolar cloud and a welldefined value for their abundance when the minerals to be
dated were formed. Such results would make these isotopes
valid chronometers.
These conclusions, however, rely on the assumption that
the abundances in the early solar nebula are those of the
steady-state ISM. This is probably true for the longer-lived
isotopes such as 92Nb, 146Sm, and 244Pu but possibly also
53Mn (e.g., Meyer and Clayton, 2000); however, it is not
for 26Al, 36Cl, 41Ca, and 60Fe. These isotopes had ESS abundances greater than those expected from a steady-state ISM.
A plausible explanation for the high abundance of 26Al,
36Cl, 41Ca, and 60Fe is that the bulk of these isotopes in the
early solar nebula were injected by a supernova that may
have triggered the collapse of the solar cloud (e.g., Cameron
and Truran, 1977; Cameron et al., 1995; Meyer and Clayton, 2000) or were injected directly into the protoplanetary
disk (Ouellette et al., 2005). Remarkably, such a supernova
could also have injected significant amounts of 129I and
182 Hf as well (e.g., Meyer, 2005). The actual manner in
which the short-lived radionuclides were injected remains
to be worked out. Therefore, it is conceivable that the injection process could have given rise to an inhomogeneous
distribution of these radionuclides. Such a result could cloud
their use as chronometers; however, the lack of collateral
anomalies in stable isotopes argues against such an inhomogeneous injection (Nichols et al., 1999).
3.2.
Local Production
Production of the isotopes of elements lighter than Fe
via irradiation of matter in the solar accretion disk by highenergy particles (>MeV) has long been considered as a
promising alternative path to stellar nucleosynthesis (Fowler
et al., 1962). The discovery that 26Al was alive in the solar
accretion disk (Lee et al., 1976) prompted studies examining its production via proton irradiation (e.g., Heymann and
Dziczkaniec, 1976; Lee, 1978). These authors estimated that
large fluences were needed to reproduce the observed 26Al
abundance. In the absence of experimental evidence for elevated fluences, irradiation failed to be considered as a viable means of producing short-lived radionuclides until recently revived by Lee et al. (1998).
At present, experimental results support the possibility
that some short-lived radionuclides were produced by irradiation. First, it is now well established that virtually all lowmass stars display an enhanced X-ray activity (Feigelson
et al., 2002; Wolk et al., 2005). Radio observations of young
stellar objects (YSOs) have resulted in the direct detection
of gyrosynchrotron radiation from MeV electrons (Güdel,
2002). YSOs also show hard X-ray spectra associated with
violent magnetic reconnection flares and baryon acceleration
at energies up to hundreds of MeV/A (Wolk et al., 2005). A
second evidence in favor of irradiation within the ESS could
be the ubiquitous presence of 10Be at the time of formation
of CAIs (McKeegan et al., 2000; Marhas et al., 2002; MacPherson et al., 2003) and the possible presence of 7Be at the
time of formation of the Allende CAI 3529-41 (Chaussidon
et al., 2006).
Trapping of GCRs in the protosolar molecular cloud core
likely contributed at some level to the initial abundance of
10Be in the ESS and possibly accounted for all of it (Desch
et al., 2004), but GCRs are not likely to have contributed
significantly to other radionuclides. For example, <0.1% of
the solar system’s initial abundance of 26Al is attributable
to GCRs (Desch et al., 2004). While this alternative origin
is a viable one for 10Be, it is also feasible that some or all of
it has an irradiation origin within the solar system (McKeegan et al., 2000; Gounelle et al., 2001; Marhas and Goswami, 2004). The very short half-life of 7Be (T½ = 53 d)
precludes its origin outside the solar system. Therefore, if
its presence can be confirmed by further analyses, it would
establish a definitive proof of irradiation within the ESS.
In any irradiation model, the main parameters are the
proton flux, the irradiation duration, the abundance of heavier cosmic rays (3He, 4He) relative to protons, the target
abundance, the nuclear cross sections, and the energy spectrum of protons (Chaussidon and Gounelle, 2006). What
mainly distinguishes the different irradiation models are
(1) the astrophysical context of irradiation, (2) the physical
nature of the targets (solid or gaseous), (3) the chemistry
of the targets, and (4) the location of the irradiated targets
relative to the source of the cosmic rays. The proton energy spectrum is usually considered to satisfy a power law,
N(E) ~ E–p, with varying index p. Models considering irradiation in the context of the progenitor molecular cloud
(e.g., Clayton and Jin, 1995) failed at reproducing the observed abundances of 26Al, 41Ca, and 53Mn, and have now
fallen into abeyance. As such, the most likely astrophysical
context for irradiation synthesis of short-lived radionuclides
is the solar accretion disk. In this context, it is recognized
that the Sun’s magnetic activity consists of two broad classes
of events. Gradual events emitting soft X-rays are electronand 3He-poor. More frequent impulsive events emit hard Xrays and are electron- and 3He-rich (Reames, 1995). Impulsive flares have steeper energy spectra than gradual flares.
Goswami et al. (2001), followed by Marhas et al. (2002)
and Marhas and Goswami (2004), developed models examining the possibility of producing short-lived radionuclides by irradiation of solar system dust by proton and 4He
nuclei at asteroidal distances. In these studies, shielding of
the whole solar accretion disk is neglected and accelerated
solar particles are supposed to have free access to dust at
~3 AU. These authors limited their models to examining
gradual events having shallow proton energy spectra (p < 3)
and normalized their yields to 10Be. Based on these models, Marhas and Goswami (2004) contended that irradiation
could account for all the 10Be, 10–20% of 41Ca and 53Mn,
and none of the 26Al inferred to be present in the ESS.
Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
Lee et al. (1998) first examined the possibility that some
short-lived radionuclides could be synthesized by irradiation in the context of the X-wind theory of low-mass star
formation (Shu et al., 1996), introducing three important
conceptual modifications compared to previous models:
(1) Irradiation takes place close to the Sun, in a gas-poor
region (i.e., the reconnection ring where magnetic lines
tying the protostar and the accretion disks reconnect), providing a powerful mechanism for accelerating 1H, 3He, and
4He nuclei to energies up to a few tens of MeV. The X-wind
provides a natural transport mechanism from the regions
close to the Sun to asteroidal distances (Shu et al., 1996).
(2) The X-wind model has opened the 3He channel for the
production of short-lived radionuclides, enhancing the 26Al
production via the reaction 24Mg(3He,p)26Al. (3) It calculates absolute yields instead of yields relative to a given
short-lived radionuclide (such as 10Be), scaling the proton
flux to observations of X-ray protostars. In this model, 26Al
and 53Mn are produced at their observed abundance for parameters corresponding to impulsive events (p = 3.5, 3He/
1H = 1.4). Calcium-41 is overproduced by 2 orders of magnitude relative to its observed abundance, while 60Fe is underproduced by several orders of magnitude. Gounelle et
al. (2001) refined this model, and calculated yields of the
recently discovered 10Be (McKeegan et al., 2000). The
production of 10Be (as well as 26Al and 53Mn) was found
to agree with the observed value in the case of impulsive
events (p = 4, 3He/1H = 0.3). They also proposed that the
41Ca overproduction could be alleviated if proto-CAIs had
a layered structure (Shu et al., 2001). Using the same model
and a preliminary estimate of 7Be nuclear cross sections,
Gounelle et al. (2003) calculated a 7Be/9Be ratio of ~0.003,
at odds with the initial claim by Chaussidon et al. (2002)
of 7Be/9Be up to ~0.22 inferred in an Allende CAI. Subsequently, Chaussidon et al. (2006) revised their estimate of
the 7Be/9Be ratio to ~0.005, compatible within a factor of
2 with the X-wind model prediction. The ability of the Xwind model to produce a relatively high abundance of 7Be
despite its very short half-life is due to the high flux of accelerated particles adopted since Lee et al. (1998). This
contrasts with the otherwise similar model of Leya et al.
(2003) that invokes special conditions for the production of
7Be. The yields of 36 Cl presented in Gounelle et al. (2006)
are slightly lower than the observed value (Lin et al., 2005),
but still in line with it, given the model uncertainties (36Clproducing cross sections have not been experimentally determined). Furthermore, if the initial solar system abundance
of 26Al was indeed supercanonical (Young et al., 2005), the
initial 41Ca/40Ca ratio was probably higher than 1.5 × 10 –8.
Therefore, a layered structure of CAIs may no longer be
required to account for the 41Ca abundance (Gounelle et al.,
2006). The decoupling of 10Be and 26Al observed in some
hibonites (Marhas et al., 2002) may be accounted for by
irradiation during gradual events instead of impulsive ones
(Gounelle et al., 2006).
To summarize, it is recognized that irradiation at asteroidal distances using “normal” proton fluences and gradual
841
events fails to produce the observed amount of short-lived
radionuclides such as 26Al, 41Ca, and 53Mn (Goswami et al.,
2001). However, there is strong evidence that the proton fluence of protostars was 105× higher than at present (Feigelson et al., 2002), and that there was intensive radial transport from the inner disk to larger heliocentric distances
(Wooden et al., 2004), either via turbulence (Cuzzi et al.,
2003) or by X-wind (Shu et al., 1996). The specific irradiation model developed in the context of the X-wind theory
can reproduce the observed abundances of 7Be, 10Be, 26Al,
36 Cl, 41Ca, and 53Mn within uncertainties for cosmic-ray
parameters corresponding to impulsive events. X-ray observations have shown that the impulsive phase is often present
in YSOs (Wolk et al., 2005).
Uncertainties of the model arise mainly from poor
knowledge of the nuclear cross sections, especially for the
3He channel. To reduce these uncertainties, nuclear physicists based at Orsay have undertaken the measurement of
3He-induced cross sections (Fitoussi et al., 2004). They
found that the experimental 24Mg(3He,p)26Al cross section
is a factor of ~2 to 3 lower than the estimate of Lee et al.
(1998), but within the range of the reported uncertainties.
At present, it is not yet clear how much irradiation processes contributed to the inventory of short-lived radionuclides. X-ray observations of protostars and realistic models reproducing the observed initial abundances of a handful
of these radionuclides call for further investigation of this
possibility.
4. ASTROPHYSICAL SETTING OF THE
SOLAR PROTOPLANETARY DISK
The presence of short-lived radionuclides with half-lives
<<10 m.y. in the ESS, at initial abundances noted in Table 1,
provides a record of the dramatic processes occurring within
a few million years of the solar system’s birth. This contrasts
with the case for the relatively longer-lived radionuclides
such as 146Sm (T½ ~ 103 m.y.) and 244Pu (T½ ~ 82 m.y.). As
mentioned in the previous section, supernovae, novae, and
AGB stars maintain steady-state abundances of these radionuclides in the galaxy that are consistent with the abundances derived from meteorites, provided there is a period of
free decay on the order of ~108 yr prior to incorporation
into the solar nebula (Schramm and Wasserburg, 1970; Harper, 1996; Jacobsen, 2005). Ongoing galactic nucleosynthesis might possibly contribute to 53Mn, 107Pd, 129I, and
182Hf as well. However, the levels at which the shorter-lived
radionuclides 26Al, 41Ca, and 60Fe (and probably 36Cl) are
maintained in the galaxy are significantly lower than those
inferred from meteorites in the ESS and after a delay of
~108 yr, essentially none of these radionuclides remains in
the molecular cloud from which the solar system formed
(Harper, 1996; Wasserburg et al., 1996; Meyer and Clayton,
2000). As such, some nearby processes were creating radionuclides within ~106 yr of the birth of the solar system.
It is clear that more than one process was involved since
there is no proposed source that can simultaneously pro-
842
Protostars and Planets V
duce enough 10Be and 60Fe. Either local irradiation or trapping of GCRs might yield the observed abundance of 10Be
(see section 2.2), but both processes underproduce 60Fe by
~3 orders of magnitude (Lee et al., 1998; Leya et al., 2003).
Iron-60 can be produced at the levels inferred from meteorites (i.e., 60Fe/56Fe ratio of up to ~10 –6) only by stellar
nucleosynthetic sources, in which beryllium is destroyed
(see section 2.1.). The meteoritic data additionally suggest
separate origins. Specifically, while 26Al and 41Ca correlate
with each other in meteoritic components (Sahijpal and Goswami, 1998), the presence of 26Al is not correlated with the
presence of 10Be (Marhas et al., 2002). Two distinct sources
are therefore required: one for 10Be, and one for 60Fe. However, as is evident from discussion in the previous section,
some of the short-lived radionuclides, particularly 26Al, 36Cl,
41Ca, and perhaps 53Mn, could be produced by both sources.
The question then is what proportions of each of these radionuclides were derived from one or the other of these
sources.
The source of the 60Fe was almost certainly a massive star
that went supernova (i.e., Type II supernova). While other
types of stellar sources could produce this isotope in sufficient abundance (Table 2), injection of material into the solar nebula by an AGB star (or a rare Type 1a supernova) is
exceedingly improbable. In particular, Kastner and Myers
(1994) quantified the spatial distribution of molecular clouds
and AGB stars and estimated an upper limit to this probability of only 3 × 10–6. At any rate, an AGB star is unlikely
to produce sufficient 60Fe relative to 26Al (Tachibana and
Huss, 2003). Therefore, the most plausible source of 60Fe
in the ESS is a Type II supernova. When that supernova injected 60Fe into the material that formed the solar system, it
is also likely to have injected other short-lived radionuclides
such as 26Al, 36Cl, 41Ca, and 53Mn (Meyer and Clayton,
2000; Goswami and Vanhala, 2000; Meyer, 2005).
Therefore, while the inferred initial abundance of 60Fe
in the ESS places its formation near a massive star that went
supernova, the timing of this event and the distance from
this supernova are uncertain. The distance may have been
several parsecs from the protosolar molecular cloud core
and triggered its collapse (Cameron and Truran, 1977;
Goswami and Vanhala, 2000; Vanhala and Boss, 2002).
Alternatively, it may have occurred <1 pc away from the
protoplanetary disk (Chevalier, 2000; Ouellette et al., 2005).
Given the extreme spatial and chemical heterogeneities of
supernova ejecta, it is difficult to definitively predict the
expected abundances of short-lived radionuclides that would
be incorporated into the solar nebula. Nevertheless, a supernova is capable of producing all the short-lived radionuclides in the ESS (except for 10Be, which has plausible
alternative sources). It has been shown that injection into
the protoplanetary disk of selected shells of the supernova
can reproduce the inferred initial abundances of various
short-lived radionuclides to within a factor of ~2 (Meyer
and Clayton, 2000; Meyer, 2005; Ouelette et al., 2005).
Future work is clearly needed to determine the relative con-
tributions of local production and stellar ejecta to the abundances of short-lived radionuclides, particularly 26Al, 36Cl,
41Ca, and perhaps also 53Mn.
5.
FROM DUST TO PLANETESIMALS
5.1. Short-lived Radionuclides in Presolar Grains
and Implications for Stellar Sources of Primordial
Dust in the Solar Nebula
Since their discovery in the late 1980s, presolar grains
in primitive meteorites have proven to be powerful tools for
improving our understanding of stellar nucleosynthesis
within and dust formation around a variety of types of stars
(e.g., Bernatowicz and Zinner, 1997; Nittler, 2003; Zinner,
2003; Clayton and Nittler, 2004; Mostefaoui and Hoppe,
2004). These grains sample most of the types of stars that
have been suggested as potential sources of short-lived isotopes in the ESS, including low-mass AGB stars and Type II
supernovae. The most common types of presolar grains are
diamond, silicates, silicon carbide, graphite, and the oxides
corundum, hibonite, and spinel. The grain size of individual
diamond grains is too small for individual isotopic analysis and even in aggregate samples, the concentrations of
most elements are too low to measure. Presolar silicates
have only recently been discovered (Messenger et al., 2003;
Nguyen and Zinner, 2004; Nagashima et al., 2004). However, they are small (<1 µm) and the most common phases,
olivine and pyroxene, do not readily take up trace elements.
Most of the evidence regarding short-lived radionuclides in
presolar grains comes from silicon carbide, with additional
evidence from oxides and graphite. About 90% of presolar
SiC grains come from low-mass AGB stars and are termed
“mainstream”; 1–2%, the X-grains, have the isotopic signature of Type II supernovae. Similarly, most presolar oxides are from AGB stars and a few are from Type II supernovae.
In low-mass AGB stars, 12C and s-process products, including some short-lived nuclides, are produced in the helium shell and periodically dredged up and mixed into the
convective envelope, where they can be trapped in dust
grains that form in stellar winds as the star loses mass (Gallino et al., 1998; Busso et al., 1999). Dust grains from AGB
stars carry short-lived nuclei made throughout the AGB
phase (lasting several hundred thousand years). In Type II
supernovae, dust condenses from the envelope of a massive
star, which is thrown off by the explosion that results from
the core collapsing to a neutron star or black hole. Shortlived nuclei are made both by nuclear reactions prior to the
explosion and due to the explosion itself (e.g., Rauscher et
al., 2002). Presolar grains from both low-mass AGB stars
and Type II supernovae preserve evidence of nucleosynthesis of a number of short-lived radionuclides and we review
the evidence here.
5.1.1. Sodium-22. There are two neon components
highly enriched in 22Ne in meteorites, Ne-E(H) and Ne-
Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
E(L). Ne-E(H) is believed to represent the Ne-isotopic composition of the helium shells of AGB stars, implanted into
mainstream presolar SiC after loss of the stellar envelope
(Gallino et al., 1990). Ne-E(L) is believed to be radiogenic,
from the decay of 22Na (T½ = 2.6 yr), for which the meteoritic carrier is presolar graphite (Amari et al., 1990). A
significant fraction of presolar graphite is from supernovae,
accounting for the preservation of Ne-E(L).
5.1.2. Aluminum-26. The presolar grain record for this
radionuclide is fairly complete (Zinner, 2003, and references
therein), as both SiC and oxides tend to have high Al/Mg
ratios as well as high initial 26Al/27Al ratios. Mainstream SiC
grains tend to have initial ratios between 10–4 and 10–3, but
a few have ratios as high as 10 –2. Presolar spinel, hibonite,
and corundum from low-mass AGB stars formed with somewhat high 26Al/27Al (Zinner et al., 2005). The 26Al/27Al ratios in mainstream SiC are similar to those predicted by
AGB stellar models, but the ratios in spinel grains require
an extra production process, most likely cool-bottom processing (Zinner et al., 2005; Nollett et al., 2003).
Most X-grains were quite 26Al-rich, with initial 26Al/27Al
ratios of 0.1–0.4. Low-density graphite grains, also believed
to come from Type II supernovae, have a somewhat wider
range of initial 26Al/27Al, from 10–5 to 0.2 (Travaglio et al.,
1999). There is abundant isotopic evidence that SiC and
graphite grains do not uniformly sample the different layers
of ejecta from Type II supernovae, but the levels of 26Al can
be explained by mixing of different layers (Travaglio et al.,
1999).
5.1.3. Calcium-41. Large 41K excesses attributed to
41Ca decay have been reported in graphite from Type II supernovae (Travaglio et al., 1999) and oxide grains from lowmass AGB stars (Nittler et al., 2005). The inferred 41Ca/40Ca
ratios are 10–3–10 –2 and 10–5–5 × 10–4, respectively. Both
types of stars produced 41Ca by neutron capture on 40Ca, in
amounts consistent with those found in presolar grains.
5.1.4. Titanium-44. This isotope has a half-life of only
67 yr, yet evidence of in situ decay has been found in graphite and SiC from supernovae. In fact, the observed excesses
in daughter 44Ca are among the strongest arguments in favor
of a supernova origin for these types of grains (Amari et al.,
1992; Hoppe et al., 1996; Travaglio et al., 1999; Besmehn
and Hoppe, 2003).
5.1.5. Vanadium-49. With a half-life of only 330 d,
49V holds the record for the shortest-lived extinct radionuclide for which evidence has been found in presolar grains.
Excesses in daughter 49Ti are correlated with V/Ti ratios in
several X-type SiC grains (Hoppe and Besmehn, 2002), and
show that grain condensation occurred within a year or so of
a supernova explosion, consistent with astronomical observations of dust condensation about 500 d after the explosion of supernova 1987A (Wooden, 1997).
5.1.6. Zircon-93. This isotope has a half-life that is
long enough (2.3 m.y.) that it behaves as if it were stable
during s-process nucleosynthesis. It decays to monoisotopic
93Nb, mostly in the ejecta of low-mass stars after the AGB
843
phase has ended. A strong correlation between the elemental
abundances of zirconium and niobium in individual presolar
SiC grains measured with a synchrotron X-ray fluorescence
microprobe shows that the grains condensed with live 93Zr
(Kashiv et al., 2006).
5.1.7. Technetium-99. Technetium has no stable isotopes, but was detected in spectra of red giant stars more
than 50 yr ago (Merrill, 1952). 99Tc (T½ = 213 k.y.) lies
along the main s-process path. In fact, most 99Ru is made as
99 Tc, which subsequently decays in AGB star envelopes or
ejecta. Comparison of Ru isotope compositions of individual presolar SiC grains with models of nucleosynthesis in
AGB stars show that the grains condensed with live 99Tc in
them (Savina et al., 2004).
5.1.8. Cesium-135. Cesium is a volatile element not expected to condense into grains at high temperature, whereas
barium (135Cs decays to 135Ba) is refractory and observed
in presolar SiC. Cesium-135 (T½ = 2.3 m.y.) is produced in
fairly high abundance in AGB stars. Comparison of highprecision Ba isotope data on aggregates of presolar SiC
(Prombo et al., 1993) with stellar nucleosynthesis models
strongly suggests that when the grains condensed, cesium
remained in the gas (Lugaro et al., 2003).
5.1.9. Outlook. There are a number of other short-lived
isotopes for which records of decay could potentially be
found in presolar grains from meteorites, but all are more
difficult than the cases given above. Manganese-53 is only
made in supernovae and there are no promising host phases
for manganese. Iron-60 is made in supernovae and AGB
stars and could be searched for in iron-bearing presolar silicates. Palladium-107 is made at fairly high abundance in
supernovae and AGB stars, but no appropriate host exists
among known types of presolar grains. Samarium-146 is
made in supernovae and in small amounts in AGB stars, but
presolar SiC tends to have low Sm/Nd ratios (Yin et al.,
2005). Hafnium-182 is also made in both supernovae and
AGB stars. Hafnium as well as tungsten form carbide and
are likely to be present in presolar SiC, but both are refractory and variations in Hf/W ratios are unlikely. Finally, 205Pb
is produced in some abundance in AGB stars, but is volatile.
5.2. Formation Timescales from Dust
to Planetesimals
Evolution of the protoplanetary disk from the formation
of the smallest (millimeter- to centimeter-sized) solid objects to planetary-sized bodies was long thought to be a
broadly sequential process: CAIs representing the earliest
material are followed by chondrules and then larger objects
of asteroidal to planetary sizes. However, recent chronological studies of short-lived radionuclides and U-Pb systematics in meteorites and their components are revealing a picture that is not as orderly.
5.2.1. Formation of calcium-aluminum-rich inclusions
and chondrules. The state of isotopic chronology of chondrule and CAI formation as of mid-2004 has been thor-
844
Protostars and Planets V
oughly reviewed by Kita et al. (2005) and need not be discussed here in detail. To summarize briefly, Pb-Pb systematics in CAIs from the Efremovka carbonaceous chondrite
give an age of 4567.2 ± 0.6 Ma (Amelin et al., 2002), which
is consistent with, but more precise than, the previously
determined Pb-Pb age of 4566 ± 2 Ma for Allende CAIs
(Göpel et al., 1994; Allègre et al., 1995). This is also the
oldest absolute age date for any solid formed in the solar
system, and as such the CAIs are believed to be the earliest solids to form within the protoplanetary disk. Based
primarily on ion microprobe analyses of 26Al-26Mg systematics in individual grains within CAIs and chondrules, a
time difference of ~1–3 m.y. between CAI and chondrule
formation has been suggested (e.g., Kita et al., 2000; Huss
et al., 2001; Amelin et al., 2002). This time difference is
supported by Pb-Pb ages of CAIs from the Efremovka (reduced CV3) chondrite and chondrules from the Acfer 059
(CR) chondrite (Amelin et al., 2002).
Detailed in situ studies (by ion microprobe or laser ablation multicollector inductively coupled plasma mass spectrometer) of 26Al-26Mg systematics in CAIs (e.g., Hsu et
al., 2000; Young et al., 2005; Taylor et al., 2005) suggest a
prolonged residence, up to ~300,000 yr, of CAIs in the protoplanetary disk. In contrast, however, Mg-isotopic analyses of “bulk” CAIs appear to indicate that they were formed
within a relatively narrow time interval of ~50,000 yr (Bizzarro et al., 2004). This apparent discrepancy could be indicative of the possibility that the in situ and bulk analyses
are recording different Al/Mg fractionation events in the
history of CAI formation.
The existence of compound chondrule-CAI objects (Krot
and Keil, 2002; Itoh and Yurimoto, 2003; Krot et al., 2005a)
indicates that chondrule formation and remelting of CAIs
overlapped in time; 26Al-26Mg systematics in bulk chondrules from Allende further suggest that chondrules began
forming contemporaneously with CAIs, and then continued
to form over a time span of at least ~2–3 m.y. (Bizzarro et
al., 2004). Near-contemporaneous formation of at least some
chondrules with CAIs is additionally supported by the PbPb age of 4566.7 ± 1.0 Ma obtained for a group of Allende
chondrules (Amelin et al., 2004). However, as suggested by
Krot et al. (2005a), chronologic information derived from
bulk chondrules may reflect the timing of formation of chondrule precursor materials rather than the time of chondrule
formation.
Chondrules from metal-rich CB carbonaceous chondrites
Gujba and Hammadah al Hamra 237 have the youngest absolute age (i.e., 4562.8 ± 0.9 Ma) yet reported for chondrules from any of the unequilibrated chondrites (Krot et al.,
2005b). It is likely that these chondrules formed from a vapor-melt plume produced by a giant impact between planetary embryos after dust in the protoplanetary disk had
largely dissipated. It is inferred from these results that planetsized objects existed in the early asteroid belt ~4–5 m.y.
after the formation of CAIs.
It has been recently shown that composition of chondrule
minerals is inconsistent with crystallization from the melt
under closed-system conditions, and that gas-melt interaction must have occurred during chondrule formation
(Libourel et al., 2005). Formation of chondrules in opensystem conditions explains their compositional and structural diversity, but it also creates an additional difficulty in
dating these objects. Matching the compositional variations
in chondrules with their isotopic systematics will be the
matter of future studies.
5.2.2. Accretion and differentiation of planetesimals.
From dating of achondrites (i.e., meteorites that formed as
a result of extensive melting on their parent planetesimals)
using long-lived isotope chronometers, such as 87Rb-87Sr
(T½ ~ 56 G.y.) or 147Sm-143Nd (T½ ~ 106 G.y.), it has been
known for a long time that their parent bodies had undergone planet-wide melting and differentiation early in solar
system history (e.g., Lugmair, 1974; Allègre et al., 1975;
Nyquist et al., 1986; Wadhwa and Lugmair, 1995; Kumar
et al., 1999). However, the uncertainties of these absolute
ages were too large — typically tens of millions of years —
to really pin down the timescales at a desirable resolution.
During the last decade or so significant advances have been
made with the use of chronometers based on short-lived
radionuclides toward obtaining high-resolution timescales
of planetesimal melting and differentiation, which in turn
have helped to place limits on the timescales required to
accrete larger (tens to hundreds of kilometers in diameter)
bodies from dust-sized particles. Here we will briefly summarize some of the more significant recent results bearing
on the timescales of planetesimal accretion and differentiation. More detailed discussions on this topic may be found
in Nichols (2006) and Wadhwa et al. (2006).
Using the 53Mn-53Cr system (T½ = 3.7 m.y.), one of the
first comprehensive studies on differentiated meteorites belonging to the howardite-eucrite-diogenite (HED) group, assumed to originate from the differentiated asteroid 4 Vesta,
was published several years ago (Lugmair and Shukolyukov,
1998). It was shown that a planetesimal-wide differentiation caused the fractionation of Mn/Cr ratios in the mantle
sources of the HED meteorites and that this episode had
concluded 7.8 ± 0.8 m.y. before the formation of the LEW
86010 angrite (which serves as the time anchor for the shortlived 53Mn-53Cr chronometer). This translates to an age of
4564.8 ± 0.9 Ma for the conclusion of this Mn/Cr fractionation event on the HED parent body. This age can be compared with that of refractory inclusions (i.e., CAIs) found
in primitive chondrites (4567.2 ± 0.6 Ma) (Amelin et al.,
2002), which, as discussed earlier, are believed to be the
earliest condensates from the solar nebula. Considering the
time difference of 2.4 ± 1.1 m.y. and the time required to
assemble and melt a body the size of 4 Vesta, this clearly
demonstrates that the accretion of large objects occurred at a
very early time and at a very fast pace.
While both manganese and chromium are elements that
mainly reside in the silicate mantle and crust of a differentiated planetesimal, they generally are not very helpful when
trying to answer questions concerning silicate-metal segregation or core formation. Here a system based on another
Wadhwa et al.: Short-lived Radionuclides as High-Resolution Chronometers
now extinct radioisotope, 182Hf, that decays to 182W with a
half-live of 8.9 m.y., has proven to be very useful. While
both elements, hafnium and tungsten, are refractory, they
are strongly fractionated during silicate-metal segregation:
Hafnium remains preferentially in the silicates, while tungsten partitions mainly into the metal fraction. Measuring the
radiogenic contribution to 182W from the decay of 182Hf in
the remaining tungsten in silicate samples provides information on the timing of Hf/W fractionation in the mantle, while
the main Hf/W fractionation from a chondritic value may
have preceded the former during core formation.
The 182Hf-182W system was first applied to the HED
meteorites by Quitté et al. (2000), followed by additional
analyses by Yin et al. (2002) and Kleine et al. (2004). Using the Ste. Marguerite H chondrite as the time anchor for
the 182Hf-182W system, Kleine et al. (2004) obtained an age
for HED parent-body mantle differentiation of 4563.2 ±
1.4 Ma, which is in agreement with the differentiation age
derived from the 53Mn-53Cr system as discussed above. In
addition, combining the HED data with 182Hf-182W systematics in chondrites indicates that core formation on 4 Vesta
may have preceded mantle differentiation by about 1 m.y.
(Kleine et al., 2004).
It should be noted that the decay products of other shortlived, now extinct, radioactive isotopes have been detected
in the HED meteorites and other achondrites (e.g., basaltic
meteorites belonging to the angrite group) and also show
their antiquity. The former presence in achondrites of live
26Al (T = 0.73 m.y.) (e.g., Srinivasan et al., 1999; Nyquist
½
et al., 2003; Baker et al., 2005; Bizzarro et al., 2005b;
Spivak-Birndorf et al., 2005; Amelin et al., 2006) and 60Fe
(T½ = 1.5 Ma) (Shukolyukov and Lugmair, 1993a,b; Quitté
et al., 2005) has been clearly demonstrated. The important
aspect of these findings is that both of these nuclei can serve
as potent heat sources for melting and differentiation if their
abundances were sufficiently high and if the meteorite parent body had accreted at a very early time. In this context,
rather tight constraints on the timing of planetesimal accretion have additionally been placed by new high-precision
Pb-isotopic ages of 4566.2–4566.5 Ma of recently discovered differentiated meteorites (Baker et al., 2005; Amelin
et al., 2006), which indicate that their parent asteroids accreted and differentiated within ~1 m.y. of the formation of
CAIs, essentially contemporaneously with chondrule-forming events (Amelin et al., 2002, 2004). Taken together, these
observations suggest that CAIs, chondrules, and differentiated asteroids formed over the same, relatively short period
of time in rather complex and diverse disk environments.
One somewhat puzzling development in the last few
years has resulted from a refinement of the precision of Wisotopic analyses and application to iron meteorites (some
of which are thought to represent the cores of differentiated planetesimals). If differentiation and core formation on
the parent planetesimals of the iron meteorites occurred during the lifetime of 182Hf but after CAI formation, the expectation is that the 182W/184W ratios in these samples would be
more radiogenic than the solar system initial value inferred
845
from CAIs and chondrites but less radiogenic compared to
bulk chondrites (i.e., between –3.5 and –2ε units relative to
BSE). The earliest data on the W-isotopic compositions of
iron meteorites had shown that these samples indeed have
the lowest 182W/184W ratios of any solar system material,
with values ranging from ~–3 to –5ε units relative to BSE
(e.g., Lee and Halliday, 1995, 1996; Horan et al., 1998).
However, the precision of these earliest measurements was
insufficient to definitively ascertain whether any of the iron
meteorites had W-isotopic compositions that were resolvably lower than the initial value inferred for the solar system. More recent, higher-precision, W-isotopic analyses of
iron meteorites have shown that some iron meteorites do
indeed have 182W/184W ratios that are resolvably lower than
–3.5 (Markowski et al., 2006; Kleine et al., 2005; Qin et
al., 2005). Taken at face value, this suggests that these iron
meteorites formed (and that core formation on their parent
planetesimals took place) earlier than CAI formation. There
are, however, incompletely understood and possibly significant effects in the 182Hf-182W system in iron meteorites resulting from long exposure to GCRs, which may results in
an apparent lowering of the measured 182W/184W ratios in
these samples. In fact, recent results demonstrate that GCR
exposure could indeed account for the least-radiogenic Wisotopic compositions in iron meteorites, but the effect on
the 182W/184W ratio due to irradiation is unlikely to be significantly larger than ~0.5ε units (Markowski et al., 2006;
Qin et al., 2005). As such, this leaves us with the conclusion
stated earlier that formation of CAIs and chondrules on the
one hand and the accretion and differentiation of planetesimals on the other occurred within a very short time span of
perhaps no more than a couple of million years.
6.
OUTLOOK AND FINAL REMARKS
The emerging picture of the protoplanetary disk and the
potentially complex and spatially and temporally diverse
environments within it leaves many open questions, and
poses challenges for astronomers, astrophysicists, disk modelers, cosmochemists, and petrologists. The following are
some of the topics that need more detailed exploration.
1. Because of the growing evidence that the short-lived
radionuclides in the protoplanetary disk came from multiple
sources, we cannot a priori assume homogeneous distribution for any such radionuclide in the protoplanetary disk.
The relatively longer lived of these short-lived nuclides that
are produced by stellar nucleosynthesis, such as 146Sm,
244Pu, and possibly 129I and 182Hf, may be mixtures of material present in the presolar molecular cloud, and freshly
synthesized material injected into the disk by a nearby supernova. The presence of 10Be in the ESS may be due to
irradiation from the young Sun [some other short-lived radionuclides may also have been produced in this manner
(Gounelle et al., 2001)] and/or from trapping of GCRs in
the protosolar molecular cloud (Desch et al., 2004). These
sources are not mutually exclusive; for example, 26Al, the
most widely used short-lived chronometer nuclide, can be
846
Protostars and Planets V
a mixture of material from stellar sources and irradiation
mechanisms. The extent of heterogeneity in distribution of
short-lived nuclides has to be evaluated by extensive comparative studies of various ESS materials: CAIs, chondrules
of various origins (nebular and asteroidal/planetary), and
differentiated meteorites, with a set of short-lived and longlived (U-Pb) isotopic systems. “Mapping” an extinct-nuclide
chronometer onto the absolute timescale may work for a
certain nuclide in a certain group of meteorites (e.g., 53Mn53Cr in the angrites and the HED meteorites) that come from
a single parent asteroid or a homogeneous population of asteroids. We cannot assume, however, that the same chronometer would give compatible results for chondrules and
CAIs, because short-lived radionuclides could be heterogeneously distributed at this scale (Gounelle and Russell,
2005a,b).
2. A related question is the timing of injection of radionuclides into the protoplanetary disk. At what point during
its evolution did the disk encounter collision with the supernova ejecta? This question could potentially be addressed
by establishing a correlation between (1) isotopic anomalies of nucleosynthetic origin in meteoritic components;
(2) the abundances of short-lived radionuclides, e.g., 26Al,
60Fe, and 41Ca, in these materials; and (3) their absolute ages.
This may be achieved by comparative studies of U-Pb and
short-lived isotopic systems in CAIs and other refractory materials (e.g., hibonite grains from CM chondrites).
3. Based on the emerging picture of early accretion (i.e.,
within ~1 m.y. of CAI formation) of the differentiated planetesimals, it seems plausible that the accretion of undifferentiated chondrite parent asteroids occurred late (unless
such bodies were extremely small), because otherwise large
asteroids would have melted as a result of heat generated by
26Al and 60Fe decay. How primitive materials such as presolar grains, CAIs, and chondrules could survive a few million years in the disk (i.e., until they were accreted into
chondrite parent bodies) alongside the accreting and differentiating asteroids is still unclear. A better understanding
of the dynamics within a protoplanetary disk would help
to clarify this.
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